Latch Spring Keeps on Breaking and Breaking and...

Where can I get a reliable gravity inverter?

I assume the lowest bid builder used the cheapest door and associated hardware available. Cheap does not equal Good. Sorry about your luck, but you get what you pay for.

I like the repair idea using pipe insulation foam. I think it would be less expensive than my idea - installing a gravity inverter to hold the latch in the up position.

When the latch spring broke on my sliding glass door, manufactured by an obscure, long-defunct company, I substituted a chunk of foam cut from a piece of pipe insulation sleeve. That was in 1994; it still works fine in 2009.

To Jolly Jeff - We put 12 men on the moon. How many did Australia put there? None.

How about abandoning the original design and coming up with a different means to latch the door. What we did in one house that we were renting years ago, in which the patio door lock never really worked, was to get a piece of half-inch steel water pipe that was cut to just fit in the bottom track, which would very effectively keep the door from opening. We used the heavy pipe because a swift kick on the door could make a lighter piece of pipe jump out, and the door could then be opened. It was cheap, easy, and it worked well.
As fpr the door's latch spring? probably it should have been designed to have a whole turn extra, so as to work with a much lower strain level. But "value engineering" took the extra loop out of the spring. OR quite likely the heat treat was not right for that batch, and all ofg the springs were glass hard.

It makes this Oz engineer wonder how you lot ever achieved putting anybody on the moon!!!

Have you contacted the manufacturer to see if there is a technical bulletin that covers this problem? Also, without knowing how the spring that's breaking interfaces with the latch, it is hard to offer any suggestion. Maybe a picture or sketch?